What size socket do you need to remove lawnmower motor and blade?

it depends on the lawn mower, I have found them to generally be be between 17mm and 21mm. for the blades off the disc, and between 12mm and 14mm for the blades to the disc

Anyone that asks a question like this has no mechanical smarts and should not be playing with tools!

You think every lawnmower on the planet uses the same size nut on the shaft?

It might help if you mentioned the make and model of the lawnmower......

If you don't know, try the various sockets in your tool chest until you come across the one that works.

I would probably use a half inch breaker bar on the blade jammed with a 2x4. And a 3/8 socket set to remove the motor. Some lawn mowers are metric while others are inch. Whatever fits snugly.

9/16ths